Fuel cells convert a fuel and an oxidizing agent into electricity, heat and water at two spatially separated electrodes. Hydrogen or a hydrogen-rich gas may be used as the fuel and oxygen or air may be used as the oxidizing agent. The energy conversion process in the fuel cell is distinguished by particularly high efficiency. For this reason, fuel cells are gaining increasing importance for mobile, stationary and portable applications.
The polymer electrolyte membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC, a variation of the PEMFC, powered directly by methanol instead of hydrogen) are suitable for use as energy converting devices due to their compact design, their power density and high efficiency. The technology of fuel cells is broadly described in the literature, see for example K. Kordesch and G. Simader, “Fuel Cells and its Applications,” VCH Verlag Chemie, Weinheim (Germany) 1996.
The basic element of a fuel cell is a membrane-electrode-assembly (MEA). This comprises a membrane consisting of a proton-conducting polymer. This polymer material will be referenced herein as an “ionomer resin,” and it may be used to form an ionomer membrane. In a fuel cell, the opposing faces of the electrolyte-membrane are in contact with catalyst layers, which catalyze the electrochemical reactions. One of the catalyst layers forms the anode, and the other catalyst layer forms the cathode of the membrane-electrode-assembly. At the anode, hydrogen is oxidized, and at the cathode, oxygen reacts with the protons that have travelled through the ionomer membrane to yield water and electricity.
A catalyst-coated membrane (hereinafter abbreviated “CCM”) comprises a polymer electrolyte membrane that is provided on both sides with a catalytically active layer. One of these layers takes the form of an anode for the oxidation of hydrogen and the second layer takes the form of a cathode for the reduction of oxygen. As the CCM consists of three layers (anode catalyst layer, ionomer membrane and cathode catalyst layer), it is often referred to as a “three-layer MEA.”
Gas diffusion layers (GDLs), sometimes referred to as gas diffusion substrates or backings, are placed onto the anode and cathode layers to bring the gaseous reaction media (hydrogen and air) to the catalytically active layers and, at the same time, to establish an electrical contact. GDLs usually consist of carbon-based substrates, such as carbon fiber paper or woven carbon fabric, which are highly porous and provide the reaction gases with good access to the catalyst layers. Furthermore, the gas diffusion layers must be able to supply humidifying water to the anode and to remove reaction water from the cathode, preventing their system of pores from becoming blocked by flooding with water. In order to avoid flooding of the pores of the gas diffusion layers, the GDLs are impregnated with hydrophobic polymers, e.g., with polytetrafluoroethylene (PTFE). GDLs can be tailored specifically into anode-type GDLs or cathode-type GDLs, depending on into which side they are built in a MEA.
The carbon substrates from which the GDLs are manufactured exhibit a quite coarse surface. Therefore, in order to improve the contact of the GDLs to the catalyst layers of the fuel cell, the GDLs can be coated with a microlayer. The microlayer usually consists of a mixture of electrically conductive carbon black and a hydrophobic polymer, e.g., polytetrafluoroethylene (PTFE) and smoothes the coarse surface structure of the carbon substrates.
As described above, a membrane-electrode-assembly consists of five layers: a central polymer electrolyte membrane, two catalyst layers and two gas diffusion layers. The polymer electrolyte membrane consists of proton-conducting polymer materials. These materials form the ionomer membranes. Tetrafluoroethylene-fluorovinyl-ether copolymer with sulfonic acid groups is preferably used. This material is marketed by, for example, E. I. DuPont under the trade name Nafion®. However, other, especially fluorine-free, ionomer materials such as sulfonated polyether ketones or aryl ketones or polybenzimidazoles may also be used. Suitable ionomer materials are described by O. Savadogo in “Journal of New Materials for Electrochemical Systems” I, 47-66 (1998). For use in fuel cells, these membranes generally have a thickness of between 10 and 200 μm. Additionally, the surface of polymer electrolyte membranes is typically hydrophilic; however, advanced materials with hydrophobic surfaces are also known.
The anode and cathode catalyst layers contain electrocatalysts, which catalyze the respective reaction (oxidation of hydrogen at the anode and reduction of oxygen at the cathode). Preferably, the metals of the platinum group of the periodic table are used as the catalytically active components. For the most part, supported catalysts are used in which the catalytically active platinum group metals have been fixed in nano-sized particle form to the surface of a conductive support material. The average particle size of the platinum group metal is between about 1 and 10 nm. Carbon blacks with particle sizes of 10 to 100 nm and high electrical conductivity have proven to be suitable as support materials.
Various ways of manufacturing a complete membrane-electrode-assembly (MEA) have been disclosed. For example, the electrolyte membrane can first be coated on both sides with the requisite catalyst layers yielding a catalyst-coated membrane (CCM). To produce a membrane electrode assembly therefrom, GDLs need to be placed on top of the catalyst layers and laminated thereto. Alternatively, the catalyst layers can be coated first onto the gas diffusion layers to yield catalyst coated backings (CCBs). An electrolyte membrane is then placed between two catalyst coated backings and a firm contact between all three components is established by applying heat and pressure.
Thus, a MEA can be manufactured by combining a CCM (catalyst-coated membrane) with two GDLs (on the anode and the cathode side), or alternatively, by combining an ionomer membrane with two catalyst-coated backings (CCBs) at the anode and the cathode sides. In both cases, a five-layer MEA product is obtained. These two manufacturing schemes may be combined, if suitable.
In order to produce a CCB, one may use a catalyst ink. A catalyst ink is a pasty substance comprising an electrocatalyst, an ionomer, solvents and optionally other ingredients, e.g., hydrophobic polymer binders, pore-forming agents, etc. This ink is then applied using a suitable technique to the surface of the gas diffusion layer and cured by heating. The thus prepared catalyst-coated backings (CCBs) can be combined with an ionomer membrane to form a membrane-electrode-assembly.
The solvents used for preparing the ink usually comprise water and organic solvents. Depending on the amount of water, one can distinguish water-based inks, wherein water forms the major part of the solvents used, from inks wherein organic solvents form the major part.
The use of catalyst inks is well known to persons skilled in the art. For example, U.S. Pat. No. 5,869,416 discloses catalyst inks that are based predominantly on organic solvents such as propylene carbonate, ethylene carbamate and the like. However, water-based inks are mostly preferred because they are not subject to stringent occupational safety and health standards.
In U.S. Pat. No. 4,229,490, a method for catalyst application to an electrode substrate is proposed, using a catalyst ink that contains Pt black, graphite, PTFE, water and Triton-X as a surfactant. Due to the high boiling point and low vapor pressure of the Triton-X, separate washing and rinsing steps need to be applied in order to remove the surfactant after printing and drying.
U.S. Pat. No. 5,211,984 describes catalyst layers prepared using polyvinyl alcohol (PVA). The surfactant nature of the PVA provides for adequate dispersion among the supported catalyst particles in an aqueous solution and the molecular structure acts to bind the carbon particles and Nafion® agglomerates so that strong films are obtained with low weight fractions of PVA. Unfortunately, PVA is a polymer material that must be decomposed by heat or washed by water to remove it from the catalyst layer.
U.S. Pat. No. 6,127,059 describes the use of Triton-X 100 surfactant for preparing an ink comprising carbon black and polytetrafluoroethylene (PTFE). This ink is used to coat a carbon cloth substrate. Thereafter, the cloth is dried and heated for 30 minutes at 370° C. to melt the PTFE and, at the same time, to decompose and to remove the surfactant.
European Patent Application Serial No. 0 731 520 A1 describes a catalyst ink comprising an electrocatalyst, ionomer and water as a solvent. Apart from the ionomer, the ink comprises no further organic components. German Patent Application Serial No. DE 100 37 074 A1 discloses a catalyst ink comprising an electrocatalyst, an ionomer, water and an organic solvent wherein the organic solvent is at least one compound selected from linear di-alcohols with a flashpoint above 100° C. that are present in the ink in an amount ranging from 1 to 50 wt. % relative to the weight of the water.
When trying to coat hydrophobic substrates, such as, e.g., backings or polymer films, with hydrophilic water-based inks, a severe wetting problem arises especially when the coating needs to be applied in a large format. The printed ink tends to accumulate and forms islands so that several consecutive coating passes are necessary to achieve a uniform coating. This is both time-consuming and costly.
Based on the foregoing, there is a need in the art for a means for coating hydrophobic backings and other substrates with water-based inks without the aforementioned wetting problems just described. Therefore, the present invention is directed to a process for manufacturing catalyst-coated substrates with a water-based catalyst ink without the necessity to apply several coating passes in order to overcome the water repellent property of the hydrophobic surface of the substrate. The present invention is also directed to providing suitable catalyst inks for this process.
For a better understanding of the present invention together with other and further advantages and embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.